This invention relates generally to feed-forward circuits, and more particularly, to feed-forward circuits used in the amplification of high-frequency signals in television applications. In recent years, cable television systems have been required to handle an increasing number of channels. Distortion and cross-modulation between the channels limits the number of amplifiers that can be cascaded and the maximum spacing between amplifiers.
A feed-forward technique is commonly used to reduce the distortion in individual amplifiers by a factor of at least one hundred, over relatively wide bandwidths. The basic technique involves the use of two cancellation loops. In a signal cancellation loop, the distorted signal from a main amplifier is subtractively combined with a delayed form of the original signal, to produce a signal containing substantially only error components. In the second cancellation loop, the error signal derived from the signal cancellation loop is further amplified, and fed forward to be subtractively combined with the output of the main amplifier. The error components cancel and a distortion-free signal remains.
While the theory of feed-forward error cancellation is relatively simple and well known, it is not a simple matter to construct a practical device in which amplitudes and phase angles are controlled to the required degree of accuracy. To achieve cancellation of the error signals by a factor of 25 decibels (dB), the amplitudes have to be matched to within 0.5 dB and the phase angles matched to within 3.2.degree..
In the past, feed-forward amplifiers for this purpose have included separate amplifiers, and delay lines made from discrete inductances and capacitances. These discrete-component delay lines cannot be reliably reproduced, and are costly to assemble. The amplifiers used in the past have been hybrid designs, with the delay lines being plug-in components. The relatively large number of variable components in this combination resulted in a tuning procedure that was considerably flexible, but also very sensitive.
The standard amplifier modules had nonlinear phase characteristics, which had to be matched by some form of adjustable phase equalization made up of LC (inductor-capacitor) low-pass filters. One attempt at designing a linear phase amplifier was described by R. G. Meyer et al., "A Wide-band Feed-forward Amplifier," IEEE Journal of Solid State Circuits, Vol. SC-9, No. 6, December 1974. However, this single-ended design would not meet the necessary performance requirements.
Discrete designs of the past have required considerable space and were difficult to isolate from surrounding circuitry. Amplifiers of this type are consequently costly, inconvenient to manufacture and tune, and have poor temperature stability. It will therefore be appreciated that there is a real need for a low-cost feed-forward amplifier that can be manufactured and tuned reliably and conveniently, and that has good performance characteristics, including temperature stability. The present invention is directed to these ends.